If a narrow beam of light is
passed through a colloid, its path is visible. This is not possible with true
solutions. The visible cone seen is due to light-scattering action by the
colloidal particles and is known as the Faraday-Tindall effect.
The Faraday-Tindall effect is
employed in the ultramicroscope and electron microscope. In the
ultramicroscope an intense beam of light is passed through a sol against a dark
background at right angle to the plane of observation. The particles are observes
as bright spots and can be counted. For Better resolution, an electron
microscope is used and very minute particles (molecular level) can be observed
in terms of size, shape, and structure. The electron microscope uses a beam of
high-energy electrons instead of the normal light.
The light scattering property is
also used to determine the concentration of sol through measurement of
turbidity, a phenomenon known as turbidimetry. Turbidity, τ, is a fractional
decrease in intensity due to scattering as the incident light passes through a
solution.
τ = 1 ln Is
l I
Where l = length of dispersion through which the light
passes
Is = Light scattered in all directions
I
= Intensity of incident light
If the turbidity of suspension of
known concentration is determined, the concentration of a certain value can be
determined.
We can also use turbidity to
measure molecular weight.
Hc / τ = 1/M +2Bc Where H and B are constants of a particular system,
c
is concentration and M is the molecular weight.
